The present invention relates to a data communication system and in particular to processing which is carried out on the occurrence of a time-out in an on-line system having a high rank unit, such as a central computer, and a plurality of terminal units connected to the high rank unit via a communication line.
Such a system is frequently used to process enquiry and renewal operations in the bank window business, inventory management business, railway or aviation seat reservation business or the like. In general, terminal units for inputting enquiries or outputting the processed data are installed at positions which are remote from the central computer for extracting data from various files and updating data. The terminal units are connected to the central computer via a communication line. The input and output data is processed on line.
When the data link for a terminal unit in such a conventional system is not established in a time period defined by the system, although the terminal unit is waiting for the data transmission, the terminal unit interprets that as the occurrence of a time-out, cancels the control status such as the sending of a wait condition, and displays an operator message "line down" on a display device or the like of the terminal unit in order to inform the operator of the occurrence of a time-out and urge the operator to initiate maintenance and investigation of the communication line system.
Causes for the time-out are as follows:
(1) The on-line work of the central computer is being held in abeyance.
(2) The central computer or the terminal unit is out of order.
(3) The power to the modem or branch unit is off.
(4) The communication line including the modem or branch unit is out of order.
(5) A data link between the central computer and another terminal unit is busy.
(6) The on-line program does not start the on-line operation for the terminal unit concerned, but starts the on-line operation for another terminal unit.
For time-outs incurred by causes (1) to (4), it is necessary to carry out maintenance and investigation of the communication line system. Causes (5) and (6) originate from problems existing in the on-line program. Maintenance and investigation for these causes are in fact unnecessary. When time-out occurs in the conventional terminal unit as described before, however, sending wait conditions or the like are cancelled and maintenance and investigation of the communication line system are carried out irrespective of that cause. Thus, the data communication system according to the prior art has the drawback that wasteful maintenance and investigation work are carried out for a time-out incurred by the cause (5) or (6).